Brushless DC motors without permanent magnets often are interchangeably referred to as switched reluctance (SR) or variable reluctance (VR) motors. Reference to a VR motor herein is intended to include both terminologies. A VR motor has two sets of salient poles, one set on the stator which has phase windings around the poles and another set on the rotor which has no windings. The stator phase windings are sequentially energized with current pulses to rotate the rotor which is connected to a shaft output. The stator phase windings are sequenced, or commutated, by signals from a rotor position sensor. The rotor position sensing means may comprise optical sensors or magnetic sensors of the Hall effect type. The sensors typically are mounted in fixed position on the stator or motor housing adjacent the path of rotation of the rotor, and the sensed means are fixed for rotation with the rotor.
In a typical three-phase, VR motor, three Hall effect sensors may be located 120.degree. arcuately apart, centered about the rotor shaft, and are fixed either directly to the stator or to some fixture which locates them according to some known relationship with respect to the stator. A magnetic ring with four North regions and four South regions alternating in 45.degree. radial arcs of the ring are attached to the rotor or rotor shaft and serve as sensed means so that when the rotor rotates, sensor output signals can be used to directly commutate, i.e., cut on and off, the current to each of the motor phase windings as they locate each and every pole alignment.
VR motors have been proposed for driving the individual spindle assemblies of a textile yarn ring spinning frame. In such spindle assemblies, the rotor of the motor is mounted on the spindle shaft which supportably rotates a yarn collection member, such as a bobbin, during the spinning operation. A ring rail with ring and traveler reciprocates vertically along the support bobbin to wind the yarn package. The lower end of the spindle support shaft is supported for rotation in a bolster section which has an outer housing mounted in fixed position to a spindle assembly support rail of the spinning frame. The stator of the VR motor is disposed in surrounding relation to the rotor and is mounted in fixed position in a housing supportably attached in suitable manner to the bolster housing or support rail of the ring spinning frame.
It is known to provide control systems for adjusting various parameters of motor operation of a VR motor, such as speed, torque, phase commutation, phase advance, and efficiency of the motor. Certain of such systems employ analog or digital memory to store optimum control parameters relating to switching angles to demand speed and operating torque. Certain other control systems employ theoretical equations derived to predict optimum phase advance as a function of speed.
U.S. Pat. No. 4,835,448 discloses a method by which a phase advance control signal is used to control motor torque. The phase advance signal may be adjusted to provide an optimum efficiency of the motor.
U.S. Pat. No. 4,954,764 discloses a method and circuit for power efficiency improvements where a PWM waveform is adjusted to provide minimum current to an AC induction motor. Several PWM waveforms are used to operate the motor and the resulting current values are stored in memory. This sequence is repeated until a minimum current value is found according to variation of loading of the motor. This technique dynamically seeks the optimum operating point of the motor and uses current measuring equipment to determine actual efficiency.
U.S. Pat. No. 4,942,344 describes apparatus and method for controlling the amount of torque angle shift in a brushless motor. The stator winding voltage level is continuously monitored to determine the amount of torque angle shift to be implemented. This technique dynamically seeks the optimum operating point of the motor and uses stator voltage measuring equipment to determine actual efficiency.